The present invention relates generally to systems of the "plate" type for osteosynthesis of the spine.
Systems of this kind are known in the art and include at least two pedicular screws anchored in adjacent vertebrae to be treated and a connecting plate designed to connect the screwheads together in a rigid manner.
EP-A-0 441 084 in particular discloses a system of this kind in which each pedicular screw has a hexagonal section part for inserting the screw into the bone and on top of which is a threaded shank.
The plate has a plurality of oblong openings through which the various threaded shanks can be inserted and an open groove on its bottom face to prevent rotation of the hexagonal part of each pedicular screw, in order to prevent it coming loose.
Each pedicular screw is associated with a stirrup through which the threaded shank of the screw also passes and which straddles the top of the plate; finally, a nut is screwed onto the threaded shank to trap and immobilize the plate with the stirrup on top of it, between it and the hexagonal part of the screw.
Complementary raised patterns are provided on the top face of the plate and on the bottom face of the stirrup to prevent longitudinal sliding of the plate relative to the screw.
Although it is generally satisfactory, this fixing system nevertheless has certain drawbacks.
The raised patterns have to be provided to prevent sliding because the plate and the stirrup cooperate only via two plane faces in compression and so their absence would lead to the risk of entirely unacceptable relative movement of the vertebrae.
The machining required by these raised patterns significantly increases the unit cost of the plates and the stirrups.
Also, the raised patterns can impede fine adjustment of the system in that they allow only a particular number of discrete mutual positions of the plate and the stirrup, i.e. a particular number of discrete distances between the screws; moreover, if the nut is overtightened before the final tightening, mutual sliding of the plate and the stirrup during adjustments may be impeded.
Finally, the above prior art system requires the various stirrups to be attached to the top of the plate, before screwing on the nuts, which is irksome and entails the risk of incorrect positioning of the stirrups before tightening.